Circuits to produce an indication or output signal in synchronism with an input signal are well known. For example, the horizontal sweep trigger circuit of an oscilloscope performs such a function based upon the input signal achieving a certain amplitude or perhaps exhibiting a certain minimum rate of change. Such applications often require that the input signal be a separate signal free of nonrelated signals and have a reasonable signal to noise ratio as well.
Consider, however, the task of producing an indication or output signal in synchronism with a generally periodic complex signal amid unrelated other signals in a noisy environment. Say, for instance, one needed to isolate the sound of dripping water from a leaky faucet amid the sound of a radio playing in the background, and produce an indication synchronized with the individual drops. Or, suppose the task were to isolate, measure the frequency of, and provide a real time indication of each occurrence of a fetal heat beat within the uterus. Autocorrelation techniques are highly useful in such circumstances to provide frequency information for the signal to be isolated. However, such frequency information does not contain a real time indication of when the signal actually occurs.
The present invention deals with a process or a device for the measurement of the frequency or period of the basic oscillation of an approximately periodic signal with statistically distributed spectral components using autocorrelation.
Such a process and such a device are known from German Laid-Open Patent Application 25 46 856. Accordingly, it is possible to determine exactly the frequency or period of a frequency in real time, that is, essentially, to display its duration immediately upon termination of the period. In this way, in particular using the Doppler principle, ultrasonic signals obtained from the fetal heart beat can be employed as the basis for an accurate measurement of the heartbeat rate. The employment of the correlation technique for the improvement of the measurement of the heart rate is, however, associated with the disadvantage that, when carrying out autocorrelation, the phase information on the heartbeat is lost. In consequence neither a beat-to-beat display of the heart rate, nor a visual or acoustic beat display is possible.